Wafer to wafer bonding method and wafer to wafer bonding system

ABSTRACT

A wafer to wafer bonding method includes performing a plasma process on a bonding surface of a first wafer, pressurizing the first wafer after performing the plasma process on the bonding surface of the first wafer, and bonding the first wafer to a second wafer. The plasma process has different plasma densities along a circumferential direction about a center of the first wafer. A middle portion of the first wafer protrudes after pressurizing the first wafer. The first wafer is bonded to the second wafer by gradually joining the first wafer to the second wafer from the middle portion of the first wafer to a peripheral region of the first wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to Korean PatentApplication No. 10-2018-0090872, filed on Aug. 3, 2018 in the KoreanIntellectual Property Office (KIPO), the disclosure of which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

Exemplary embodiments relate to a wafer to wafer bonding method and awafer to wafer bonding apparatus. More particularly, exemplaryembodiments relate to method of bonding wafers to each other tomanufacture a semiconductor device having a three-dimensional connectionstructure, and a wafer to wafer bonding apparatus for performing thesame.

DISCUSSION OF THE RELATED ART

In manufacturing electronic products such as a CMOS image sensor (CIS),High Bandwidth Memory (HBM), etc., two wafers may be bonded to eachother, thereby improving a yield rate per wafer. The wafer to waferbonding process may include an O₂ plasma activation step, a hydrationstep, a wafer alignment step, a wafer bonding step, an annealing step,etc. When the two wafers are bonded to each other in the wafer bondingstep, an overlay distortion may occur between the bonded wafers.

SUMMARY

Exemplary embodiments provide a wafer to wafer bonding method capable ofpreventing wafer to wafer misalignment.

Exemplary embodiments provide a wafer to wafer bonding apparatus forperforming the wafer to wafer bonding method.

According to an exemplary embodiment, a wafer to wafer bonding methodincludes performing a plasma process on a bonding surface of a firstwafer, pressurizing the first wafer after performing the plasma processon the bonding surface of the first wafer, and bonding the first waferto a second wafer. The plasma process has different plasma densitiesalong a circumferential direction about a center of the first wafer. Amiddle portion of the first wafer protrudes after pressurizing the firstwafer. The first wafer is bonded to the second wafer by graduallyjoining the first wafer to the second wafer from the middle portion ofthe first wafer to a peripheral region of the first wafer.

According to an exemplary embodiment, a wafer to wafer bonding methodincludes performing a plasma process on a bonding surface of a firstwafer, pressurizing the first wafer after performing the plasma processon the bonding surface of the first wafer, and bonding the first waferto a second wafer. The plasma process has a first plasma density in afirst crystal orientation from a center of the first wafer and a secondplasma density in a second crystal orientation of the first wafer. Amiddle portion of the first wafer protrudes after pressurizing the firstwafer. The first wafer is bonded to the second wafer by graduallyjoining the first wafer to the second wafer from the middle portion ofthe first wafer to a peripheral region of the first wafer.

According to an exemplary embodiment, a wafer to wafer bonding systemincludes a plasma processing apparatus and a wafer bonding apparatus.The plasma processing apparatus is configured to perform a plasmaprocess on a bonding surface of a first wafer. The plasma process hasdifferent plasma densities along a circumferential direction about acenter of the first wafer. The wafer bonding apparatus is configured tobond the first wafer to a second wafer by protruding a middle portion ofthe first wafer toward the second wafer, and gradually joining the firstwafer to the second wafer from the middle portion of the first wafer toa peripheral region of the first wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will become moreapparent by describing in detail exemplary embodiments thereof withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating a wafer to wafer bonding systemaccording to exemplary embodiments.

FIG. 2 is a cross-sectional view illustrating a plasma processingapparatus in FIG. 1.

FIG. 3 is a bottom view illustrating a shower head of the plasmaprocessing apparatus in FIG. 2.

FIG. 4A is a cross-sectional view taken along line A-A′ in FIG. 3.

FIG. 4B is a cross-sectional view taken along line B-B′ in FIG. 3.

FIG. 5 is a view illustrating an adhesion force distribution on asurface of a wafer is plasma-processed using the shower head of FIG. 3.

FIG. 6 is a bottom view illustrating the shower head of the plasmaprocessing apparatus according to exemplary embodiments.

FIG. 7A is a cross-sectional view taken along line A-A′ in FIG. 6.

FIG. 7B is a cross-sectional view taken along line B-B′ in FIG. 6.

FIG. 8 is a cross-sectional view illustrating the wafer to wafer bondingapparatus in FIG. 1.

FIG. 9 is a flowchart illustrating a wafer to wafer bonding methodaccording to exemplary embodiments.

FIG. 10 is a view illustrating the wafer to wafer bonding method in FIG.9.

FIG. 11 is a view illustrating a crystal orientation of a wafer.

FIG. 12 is a graph illustrating Young's modulus with respect to thecrystal orientation in FIG. 11.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary embodiments will be described more fully hereinafter withreference to the accompanying drawings. Like reference numerals mayrefer to like elements throughout the accompanying drawings.

Spatially relative terms, such as “beneath”, “below”, “lower”, “under”,“above”, “upper”, etc., may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” or“under” other elements or features would then be oriented “above” theother elements or features. Thus, the exemplary terms “below” and“under” can encompass both an orientation of above and below.

The term “about” as used herein is inclusive of the stated value andmeans within an acceptable range of deviation for the particular valueas determined by one of ordinary skill in the art, considering themeasurement in question and the error associated with measurement of theparticular quantity (e.g., the limitations of the measurement system).For example, “about” may mean within one or more standard deviations asunderstood by one of the ordinary skill in the art. Further, it is to beunderstood that while parameters may be described herein as having“about” a certain value, according to exemplary embodiments, theparameter may be exactly the certain value or approximately the certainvalue within a measurement error as would be understood by a personhaving ordinary skill in the art.

Further, when two or more elements or values are described as beingabout equal to each other, it is to be understood that the elements orvalues are identical to each other, indistinguishable from each other,or distinguishable from each other but functionally the same as eachother as would be understood by a person having ordinary skill in theart.

FIG. 1 is a block diagram illustrating a wafer to wafer bonding systemaccording to exemplary embodiments. FIG. 2 is a cross-sectional viewillustrating a plasma processing apparatus in FIG. 1. FIG. 3 is a bottomview illustrating a shower head of the plasma processing apparatus inFIG. 2. FIG. 4A is a cross-sectional view taken along line A-A′ in FIG.3. FIG. 4B is a cross-sectional view taken along line B-B′ in FIG. 3.FIG. 5 is a view illustrating an adhesion force distribution on asurface of a wafer is plasma-processed using the shower head of FIG. 3.FIG. 6 is a bottom view illustrating the shower head of the plasmaprocessing apparatus according to exemplary embodiments. FIG. 7A is across-sectional view taken along line A-A′ in FIG. 6. FIG. 7B is across-sectional view taken along line B-B′ in FIG. 6. FIG. 8 is across-sectional view illustrating the wafer to wafer bonding apparatusin FIG. 1.

Referring to FIGS. 1 to 8, a wafer to wafer bonding system 10 mayinclude a plasma processing apparatus 40, a cleaning apparatus 50, analigning apparatus 60, and a wafer bonding apparatus 70. The wafer towafer bonding system 10 may be arranged in a clean room 20. The wafer towafer bonding system 10 may further include a cassette stage 30. Thecassette stage 30 may be disposed in a side of the clean room 20.

In exemplary embodiments, the clean room 20 may be an enclosed roomhaving a cuboid shape, and may be a controlled environment that has alow level of pollutants such as, for example, dust, airborne microbes,aerosol particles, and chemical vapors.

The cassette stage 30 may provide a space in which wafers are locatedbefore being transferred into the clean room 20. A carrier C having aplurality of the wafers received therein may be supported on a supportplate 32 of the cassette stage 30. The carrier C may be, for example, afront opening unified pod (FOUP). The wafers received in the carrier Cmay be transferred into the clean room 20 by a transfer robot 22. Forexample, three carriers C may be disposed on the cassette stage 30. Inexemplary embodiments, first and second wafers to be bonded to eachother may be received in first and second carriers C respectively, andbonded wafers may be received in a third carrier C.

In exemplary embodiments, the first wafer may be a wafer in whichcircuits for an image sensor chip are formed, and the second wafer maybe a wafer in which photosensors for the image sensor chip are formed.Alternatively, in exemplary embodiments, the first wafer may be a waferin which circuits for a semiconductor package such as a High Band Memory(HBM) are formed, and the second wafer may be a wafer in which memoriesfor the semiconductor package are formed.

The aligning apparatus 60 may detect a flat portion (or notch portion)of a wafer W to align the wafer W (see FIG. 2). The wafer aligned by thealigning apparatus 60 may be transferred to the plasma processingapparatus 40 or the wafer bonding apparatus 70 by the transfer robot 22.

As illustrated in FIG. 2, the plasma processing apparatus 40 may includea chamber 100, a substrate support 110 having a lower electrode, anupper electrode 130, and a shower head 140.

The substrate support 110 may support the wafer W within the chamber100. The substrate support 110 may include a substrate stage having thelower electrode on which the substrate is disposed.

In exemplary embodiments, the chamber 100 may be an induced coupledplasma (ICP) chamber, and the plasma processing apparatus 40 may be anapparatus configured to process plasma on a surface of the substratesuch as the wafer W disposed within the ICP chamber 100 to form adangling bond on the surface of the substrate. However, the plasmagenerated by the plasma processing apparatus 40 is not limited toinductively coupled plasma utilized in an ICP chamber. For example,exemplary embodiments may utilize capacitively coupled plasma, microwaveplasma, etc., which may be generated by the plasma processing apparatus40.

The chamber 100 may provide a sealed space in which a plasma etchprocess is performed on the wafer W. The chamber 100 may be, forexample, a cylindrically shaped vacuum chamber. The chamber 100 mayinclude a cover 102 which covers an open upper end portion of thechamber 100. The cover 102 may seal the upper end portion of the chamber100 such that it is airtight.

A gate for opening and closing a loading/unloading port of the wafer Wmay be provided in a sidewall of the chamber 100. The wafer W may beloaded/unloaded onto/from the substrate stage through the gate.

A gas exhaust port 104 may be provided in a bottom portion of thechamber 100, and a gas exhaust unit 106 may be connected to the gasexhaust port 104 through a gas exhaust line. The gas exhaust unit 106may include, for example, a vacuum pump, such as a turbo-molecular pump.The vacuum pump may control the pressure of the chamber 100 so that theprocessing space inside the chamber 100 may be depressurized to adesired vacuum level. In addition, process by-products and residualprocess gases may be discharged through the gas exhaust port 104.

The upper electrode 130 may be disposed outside the chamber 100 suchthat the upper electrode 130 faces the lower electrode. For example, theupper electrode 130 may be disposed on the cover 102. Alternatively, theupper electrode 130 may be disposed over the shower head 140 within thechamber 100 or in an upper portion of the chamber 100.

The upper electrode 130 may include a radio frequency antenna. The radiofrequency antenna may have, for example, a coil shape. The cover 102 mayinclude a dielectric window, which may have a circular plate shape. Thedielectric window may include a dielectric material. For example, thedielectric window may include aluminum oxide (Al₂O₃). Power from theantenna may be transferred into the chamber 100 through the dielectricwindow.

For example, the upper electrode 130 may include coils having a spiralshape or a concentric shape. The coil may generate plasma P (e.g.,inductively coupled plasma P) in a space of the chamber 100. It is to beunderstood that the number, arrangement, etc. of the coils may bevariously modified, and are not limited to the exemplary embodimentsdescribed herein.

Still referring to FIG. 2, in exemplary embodiments, the plasmaprocessing apparatus 40 may further include a gas supply unit connectedto the shower head 140. The gas supply unit may supply a gas into thechamber 100. The gas supply unit may include, for example, a gas supplyline 152, a flow controller 154, and a gas supply source 156, which mayinclude gas supply elements. The gas supply line 152 may be connected toan inner space 141 (see FIGS. 4A, 4B, 7A and 7B) of the shower head 140within the chamber 100.

The shower head 140 may be arranged over the substrate support 110 suchthat it faces the entire surface of the wafer W, and may spray out aplasma gas onto the surface of the wafer W through a plurality ofdischarge holes 142. The plasma gas may include a gas such as, forexample, O₂, N₂, Cl₂, etc., however, the plasma gas is not limitedthereto.

The gas supply source 156 may store the plasma gas, and the plasma gasmay be supplied into the chamber 100 through the shower head 140connected to the gas supply line 152. The flow controller 154 maycontrol an amount of the gas supplied into the chamber 100 through thegas supply line 152. The flow controller 154 may include, for example, amass flow controller (MFC).

A first power supply 131 may apply a plasma source power to the upperelectrode 130. The first power supply 131 may include a source RF powersource 134 and a source RF matcher 132 such as, for example, plasmasource elements. The source RF power source 134 may generate a radiofrequency (RF) signal. The source RF matcher 132 may match impedance ofthe RF signal generated by the source RF power source 134 using thecoils to control generation of plasma.

A second power supply 121 may apply a bias source power to the lowerelectrode. For example, the second power supply 121 may include a biasRF power source 124 and a bias RF matcher 122 such as, for example, biaselements. The bias RF power source 124 may generate an RF signal. Thebias RF matcher 122 may match impedance of the bias RF signal bycontrolling a bias voltage and bias current applied to the lowerelectrode. The bias RF power source 124 and the source RF power source134 may be synchronized or desynchronized with other through asynchronizer of a controller.

The controller may be connected to the first power supply 131 and thesecond power supply 121, and may control operations thereof. Thecontroller may include a microcomputer and various interface circuitsthat may be used to control an operation of the plasma processingapparatus 40 based on programs and recipe information stored in anexternal or internal memory.

As the radio frequency power having a predetermined frequency is appliedto the upper electrode 130, an electromagnetic field induced by theupper electrode 130 may be applied to a source gas supplied within thechamber 100 to generate the plasma P. As the bias power having apredetermined frequency less than the frequency of the plasma power isapplied to the lower electrode, plasma atoms or ions generated withinthe chamber 100 may be attracted toward the lower electrode.

In exemplary embodiments, the plasma processing apparatus 40 may performa local plasma process on a surface of the wafer W. The plasmaprocessing apparatus 40 may perform the plasma process with differentplasma densities along at least a circumferential direction about acenter with respect to the surface of the wafer W.

For example, the plasma processing apparatus 40 may perform a processwith a first plasma density in a first direction from the center O (seeFIG. 5) of the wafer W, and may perform a process with a second plasmadensity in a second direction oriented to the first direction at apredetermined angle along the circumferential direction about the centerO of the wafer W. In exemplary embodiments, the second plasma densitymay be greater than the first plasma density.

As illustrated in FIGS. 3, 4A and 4B, in exemplary embodiments, theplasma processing apparatus 40 may include the shower head 140, whichincludes a plurality of discharge holes 142. In exemplary embodiments,the discharge holes 142 may be non-uniformly disposed in the shower head140 along the circumferential direction so as to perform the localplasma process along at least the circumferential direction about thecenter O (see FIG. 5) of the wafer W.

The shower head 140 may include a plurality of regions, includingregions C1 and C2, along the circumferential region. For example, theshower head 140 may include a first region C1 in a first (radial)direction corresponding to a first crystal orientation (the X direction)of the wafer W, and a second region C2 in a second (radial) directioncorresponding to a second crystal orientation (about 45° from the Xdirection). For example, the first crystal orientation of the wafer Wmay be in a [100] direction and the second crystal direction of thewafer W may be in a [110] direction (see FIGS. 11 and 12). The firstregion C1 and the second region C2 may be arranged alternately along thecircumferential direction about the center O. Each of the first andsecond regions C1 and C2 may have a sector shape with the central angleof about 45°.

The shower head 140 may include a first group of the discharge holes 142a through which the plasma gas is sprayed out with a first dischargeamount in the first crystal orientation of the wafer W, and a secondgroup of the discharge holes 142 b through which the plasma gas issprayed out with a second discharge amount greater than the firstdischarge amount in the second crystal orientation of the wafer W. Thefirst group of the discharge holes 142 a may be formed in the firstregion C1, and the second group of the discharge holes 142 b may beformed in the second region C2.

An opening area of the second group of discharge holes 142 b may begreater than an opening area of the first group of discharge holes 142a. That is, the total amount of open area provided by the second groupof discharge holes 142 b may be greater than the total amount of openarea provided by the first group of discharge holes 142 a. In exemplaryembodiments, this relationship may exist between the first group ofdischarge holes 142 a and the second group of discharge holes 142 b inwhich the size of each discharge hole included in the first group ofdischarge holes 142 a is about equal to the size of each discharge holeincluded in the second group of discharge holes 142 b. In this case, thenumber of the second group of discharge holes 142 b may be greater thanthe number of the first group of discharge holes 142 a. That is, in thiscase, the size of all discharge holes may be the same, and there may bemore discharge holes included in the second group of discharge holes 142b than discharge holes included in the first group of discharge holes142 a. Accordingly, the shower head 140 may spray the plasma gas withthe more discharge amount in the second crystal orientation than in thefirst crystal orientation of the wafer W.

As illustrated in FIG. 5, in exemplary embodiments, a distribution of anadhesion force due to a dangling bond on the surface of the wafer W onwhich a plasma process is performed using the shower head 140 indicatesthat the adhesion force generated in a first region D1 in the firstcrystal orientation from the center O of the wafer W is different fromthe adhesion force generated in a second region D2 in the second crystalorientation. For example, it may be shown that the adhesion forcegenerated in the second crystal orientation is greater than the adhesionforce in the first crystal orientation. Accordingly, an adhesion forcein the second crystal direction between the bonded wafers may berelatively increased, and an adhesion force in the first crystaldirection between the bonded wafers may be relatively decreased.

As illustrated in FIGS. 6, 7A and 7B, the shower head 140 may includethe first group of discharge holes 142 a formed in the first region C1in the first (radial) direction from the center thereof, and the secondgroup of discharge holes 142 b formed in the second region C2 in thesecond (radial) direction oriented to the first direction at apredetermined angle (e.g., about 45°) along the circumferentialdirection about the center. The number of the second group of dischargeholes 142 b may be greater than the number of the first group ofdischarge holes 142 a. That is, in exemplary embodiments, the showerhead 140 may include more discharge holes in the second group ofdischarge holes 142 b than in the first group of discharge holes 142 a.

In addition, as shown in FIG. 6, the number of the discharge holes 142 aand 142 b in a peripheral region of the shower head 140 may be greaterthan the number of the discharge holes 142 a and 142 b in a middleregion of the shower head 140. Thus, a distribution of plasma densityalong a radial direction on the wafer W may be adjusted.

In exemplary embodiments, the distribution of the discharge holes 142 aand 142 b of the shower head 140 may be adjusted to perform the localplasma process. However, exemplary embodiments are not limited thereto.For example, in exemplary embodiments, the arrangement of the coils asthe upper electrode 130 may be adjusted to perform the local plasmaprocess.

Referring again to FIG. 1, the cleaning apparatus 50 may clean thesurface of the wafer that has been plasma-processed by the plasmaprocessing apparatus 40. The cleaning apparatus 50 may coat deionized(DI) water on the wafer surface using a spin coater. The DI water mayclean the wafer surface and allow —OH radical to be bonded easily on thewafer surface, such that dangling bonds are easily created on the wafersurface.

As illustrated in FIG. 8, the wafer bonding apparatus 70 may include alower chuck structure and an upper chuck structure. The upper chuckstructure may include an upper stage 210 that holds a first wafer W1,and the lower chuck structure may include a lower stage 200 that holds asecond wafer W2.

The first wafer W1 may be vacuum suctioned by suction holes 212 of theupper stage 210, and the second wafer W2 may be vacuum suctioned bysuction holes 202 of the lower stage 200.

The upper stage 210 may be configured such that it is movable upwardlyand downwardly by an elevating rod 222. Accordingly, the upper stage 210may move the suctioned first wafer W1 toward the second wafer W2disposed on the lower stage 200. The elevating rod 222 may be fixedlyinstalled to an upper frame 220.

The lower stage 200 may be arranged to face the upper stage 210. Thelower stage 200 may be configured such that it is movabletranslationally and rotationally such that a relative position betweenthe upper stage 210 and the lower stage 200 may be adjusted.

The wafer bonding apparatus 70 may include a push rod 230 forpressurizing a middle region of the first wafer W1. The push rod 230 maybe configured such that it is movable through the upper stage 210. Forexample, the upper stage 210 may include an opening through which thepush rod 230 may move through.

After the first and second wafers W1 and W2 are held by the upper stage210 and the lower stage 200, a wafer bonding process may be performed,as described below.

First, the first wafer W1 may be suctioned on the upper stage 210 with auniform pressure across the entire surface of the first wafer W1, andthe second wafer W2 may be suctioned on the lower stage 200 with auniform pressure across the entire surface of the second wafer W2.

Then, the push rod 230 may descend toward the lower stage 200 topressurize the middle portion of the first wafer W1. Thus, the middleportion of the first wafer W1 may protrude downward more than theperipheral region, such that the first wafer W1 is bent downward, asshown in FIG. 8. The peripheral region of the first wafer W1 may bevacuum suctioned by the suction holes 212 of the upper stage 210.

When the first wafer W1 bends downward such that it is downwardlyconcave, the upper stage 210 may travel downward such that the firstwafer W1 contacts the second wafer W2. The middle portion of the firstwafer W1 may initially contact the second wafer W2, as shown in FIG. 8,and the remaining portion of the first wafer W1 may then gradually makecontact with the second wafer W2 from the middle portion toward theperipheral region, such that the first and second wafers W1 and W2 arejoined.

For example, in exemplary embodiments, a bonding surface of the firstwafer W1 is pressurized to cause the middle portion of the first waferW1 to protrude. Then, the first wafer W1 is bonded to the second waferW2 by gradually joining the first wafer W1 to the second wafer W2 fromthe middle portion of the first wafer W1 to the peripheral region of thefirst wafer W1. The peripheral region of the first wafer W1 may refer tothe outer edges of the wafer W1. Thus, in exemplary embodiments, thefirst wafer W1 is first joined to the second wafer W2 at the middleportion of the first wafer W1 (e.g., at a location where the first waferW1 protrudes most), and is then subsequently gradually joined to thesecond wafer W2 from the middle portion of the first wafer W1 toward theedges of the first wafer W1.

Then, a vacuum pressure may be removed from the suction holes 212 of theupper stage 210 to bond the first wafer W1 and the second wafer W2 toeach other.

The wafer to wafer bonding system 10 may further include an annealingapparatus that may thermally treat the bonded wafers. In addition, thewafer to wafer bonding system 10 may further include a grindingapparatus that grinds a surface of at least one of the bonded wafers. Inthis case, the grinding apparatus may grind a surface of the wafer inwhich the photosensors are formed.

Referring to a comparative example, in a wafer bonding step, a middleregion of a first wafer may be deformed to protrude, and then may bejoined to a second wafer gradually from a middle region to a peripheralregion. As a result, after the wafers are bonded to each other,different restoring forces may be generated due to an elastic modulusdifference depending on the orientation of a crystal lattice of thefirst wafer. Thus, an overlay distortion may occur between the bondedwafers.

According to exemplary embodiments, different restoring forces to begenerated due to an elastic modulus difference depending on theorientation of the crystal lattice of the wafer between the wafersbonded to each other by the wafer bonding apparatus 70 of the wafer towafer bonding system 10 may be predicted, and the plasma processingapparatus 40 of the wafer to wafer bonding system 10 may perform thelocal plasma process corresponding to the orientation of the crystallattice in a bonding surface of the wafer. As a result, an overlaydistortion between the bonded wafers may be prevented or reduced.

Hereinafter, a wafer to wafer bonding method using the wafer to waferbonding system 10 in FIG. 1 will be described.

FIG. 9 is a flowchart illustrating a wafer to wafer bonding methodaccording to exemplary embodiments. FIG. 10 is a view illustrating thewafer to wafer bonding method in FIG. 9. FIG. 11 is a view illustratinga crystal orientation of a wafer. FIG. 12 is a graph illustratingYoung's modulus with respect to the crystal orientation in FIG. 11.

Referring to FIGS. 1, 2, 3 and 8 to 12, first, a local plasma processmay be performed on at least one of bonding surfaces of wafers to bebonded to each other (S100).

In exemplary embodiments, the wafer W may be loaded into the chamber 100of the plasma processing apparatus 40, a plasma gas may be supplied onthe wafer W through the shower head 140, and the local plasma processmay then be performed within the chamber 100.

First, the semiconductor wafer W may be loaded onto an electrostaticchuck of a substrate stage within the chamber 100. The plasma gas may beintroduced into the chamber 100 through discharge holes 142 of theshower head 140, and then, a pressure of the chamber 100 may becontrolled to a desired vacuum level by the gas exhaust unit 106.

Then, a plasma power may be applied to the upper electrode 130 togenerate plasma within the chamber 100, and a bias power may be appliedto the lower electrode to perform the plasma process.

In exemplary embodiments, to form a local plasma density on the wafer W,the plasma gas may be sprayed out with different discharge amounts alonga circumferential direction about a center of the wafer W.

The plasma gas may be sprayed out in a first crystal orientation ([100]direction in FIGS. 11 and 12) of the wafer W through the first group ofdischarge holes 142 a of the shower head 140, and the plasma gas may besprayed out in a second crystal orientation ([110] direction in FIGS. 11and 12) of the wafer W through the second group of discharge holes 142 bof the shower head 140.

In exemplary embodiments, an opening area of the second group of thedischarge holes 142 b may be greater than an opening area of the firstgroup of the discharge holes 142 a. Accordingly, the shower head 140 mayspray plasma gas having a greater discharge amount in the second crystalorientation than in the first crystal orientation of the wafer W. Thus,a plasma density in the second crystal orientation may be generated tobe greater than a plasma density in the first crystal orientation.Accordingly, an adhesion force in the second crystal orientation on thewafer W may be greater than an adhesion force in the first crystalorientation on the wafer W.

In exemplary embodiments, the surface of the wafer W that has beenplasma-processed may be cleaned. DI water may be coated on the wafersurface using a spin coater. The DI water may clean the wafer surfaceand allow —OH radical to be bonded easily on the wafer surface, suchthat dangling bonds are easily created on the wafer surface.

The above-described process may be repeated for a plurality of wafers.

Then, the plasma-processed wafers may be aligned (S110), at least one ofthe wafers may be pressurized such that a middle portion of the waferprotrudes (S120), and then, the wafers may be gradually joined from themiddle portion to a peripheral region (S130). For example, referring tooperation S130, one of the wafers may gradually make contact withanother wafer from the middle portion toward the peripheral region, asdescribed above. This process may also be referred to herein as abonding process.

In exemplary embodiments, plasma-processed first and second wafers W1and W2 may be vacuum suctioned on the upper stage 210 and the lowerstage 200 of the wafer bonding apparatus 70 (see FIG. 8). The firstwafer W1 may be vacuum suctioned by the suction holes 212 of the upperstage 210, and the second wafer W2 may be vacuum suctioned by thesuction holes 202 of the lower stage 200.

Then, the push rod 230 may descend to pressurize the middle portion ofthe first wafer W1 such that the middle portion of the first wafer W1 isbent downward. The peripheral region of the first wafer W1 may be vacuumsuctioned by the suction holes 212 of the upper stage 210.

When the first wafer W1 bends downward to be downwardly concave, theupper stage 210 may travel downward such that the first wafer W1contacts the second wafer W2. The middle portion of the first wafer W1may initially contact the second wafer W2, and the remaining portion ofthe first wafer W1 may then gradually make contact with the second waferW2 from the middle portion toward the peripheral region, such that thefirst and second wafers W1 and W2 are joined. Then, a vacuum pressuremay be removed from the suction holes 212 of the upper stage 210 to bondthe first wafer W1 and the second wafer W2 to each other.

The adhesion force in the second crystal orientation ([110] direction inFIGS. 11 and 12) between the first and second wafers W1 and W2 bonded toeach other may be greater than the adhesion force in the first crystalorientation ([100] direction in FIGS. 11 and 12).

The wafer may be formed of an anisotropic crystalline material whosematerial properties depend on orientation relative to the crystallattice. As illustrated in FIGS. 11 and 12, the wafer W may have a firstYoung's modulus Y1 in the [100] direction and a second Young's modulusY2 in the [110] direction. The second Young's modulus Y2 is greater thanthe first Young's modulus Y1. Accordingly, a strain rate in the [100]direction may be greater than a strain rate in the [110] direction.

Since the middle region of the first wafer W1 is deformed such that itprotrudes, and is then bonded to the second wafer W2, restoring forcesmay be generated on the bonding surfaces of the first and second wafersW1 and W2 bonded to each other. Since the strain rate in the firstcrystal orientation ([100] direction) is greater than the strain rate inthe second crystal orientation ([110] direction), an overlay distortionbetween the wafers W1 and W2 may occur during a subsequent annealingprocess.

According to exemplary embodiments, because the local plasma process isperformed such that the adhesion force in the second crystal orientation([110] direction) between the first and second wafers W1 and W2 isgreater than the adhesion force in the first crystal orientation ([100]direction), more deformations may be generated in the first crystalorientation relative to the second crystal orientation during theannealing process, thereby reducing or preventing the overlay distortionbetween the wafers W1 and W2.

As described above, according to exemplary embodiments, differentrestoring forces to be generated due to an elastic modulus differencedepending on the orientation of a crystal lattice of a wafer from amongwafers bonded to each other by a wafer bonding apparatus may bepredicted, and a plasma processing apparatus may perform a local plasmaprocess corresponding to the orientation of the crystal lattice in abonding surface of the wafer. As a result, an overlay distortion betweenthe bonded wafers may be prevented or reduced.

The above-described wafer to wafer bonding system and wafer to waferbonding method may be used to manufacture, for example, semiconductorpackages or image sensors including logic devices and memory devices.For example, the semiconductor packages may include volatile memorydevices such as DRAM devices and SRAM devices, or non-volatile memorydevices such as flash memory devices, PRAM devices, MRAM devices, ReRAMdevices, etc. The image sensor may include a CMOS image sensor.

While the present invention has been particularly shown and describedwith reference to the exemplary embodiments thereof, it will beunderstood by those of ordinary skill in the art that various changes inform and detail may be made therein without departing from the spiritand scope of the present invention as defined by the following claims.

What is claimed is:
 1. A wafer to wafer bonding method, comprising:performing a plasma process on a bonding surface of a first wafer bygenerating a first plasma density in a first direction from a center ofthe first wafer, and generating a second plasma density greater than thefirst plasma density in a second direction oriented to the firstdirection at a predetermined angle along a circumferential directionabout the center of the first wafer; pressurizing the first wafer afterperforming the plasma process on the bonding surface of the first wafer,wherein a middle portion of the first wafer protrudes after pressurizingthe first wafer; and bonding the first wafer to a second wafer bygradually joining the first wafer to the second wafer from the middleportion of the first wafer to a peripheral region of the first wafer. 2.The method of claim 1, wherein the first direction corresponds to afirst crystal orientation of the first wafer, and the second directioncorresponds to a second crystal orientation different from the firstcrystal orientation.
 3. The method of claim 2, wherein the first waferhas a first Young's modulus in the first crystal orientation and thesecond wafer bonded to the first wafer has a second Young's modulus inthe second crystal orientation.
 4. The method of claim 2, wherein thefirst crystal orientation is in a [100] direction and the second crystalorientation is in a [110] direction.
 5. The method of claim 1, whereinthe second direction is oriented to the first direction at about 45°along the circumferential direction about the center of the first wafer.6. The method of claim 1, wherein performing the plasma processcomprises: supplying a plasma gas onto the bonding surface of the firstwafer through a shower head, wherein the shower head comprises: a firstgroup of discharge holes through which the plasma gas is sprayed outwith a first discharge amount in the first direction; and a second groupof discharge holes through which the plasma gas is sprayed out with asecond discharge amount greater than the first discharge amount in thesecond direction.
 7. The method of claim 6, wherein a number ofdischarge holes included in the second group of discharge holes isgreater than a number of discharge holes included in the first group ofdischarge holes.
 8. The method of claim 6, wherein the second directionis oriented to the first direction at about 45° along thecircumferential direction about the center of the first wafer.
 9. Themethod of claim 6, wherein the first wafer has a first Young's modulusin a first crystal orientation of the first wafer, and the second waferbonded to the first wafer has a second Young's modulus in a secondcrystal orientation of the second wafer bonded to the first wafer,wherein the second crystal orientation is different from the firstcrystal orientation.
 10. The method of claim 6, wherein an opening areaof the second group of discharge holes is greater than an opening areaof the first group of discharge holes.
 11. The method of claim 1,further comprising: cleaning the first wafer after performing the plasmaprocess on the first wafer.
 12. The method of claim 1, whereinperforming the plasma process comprises: spraying out a plasma gas witha first discharge amount in the first direction; and spraying out theplasma gas with a second discharge amount greater than the firstdischarge amount in the second direction.
 13. The method of claim 1,further comprising: annealing the first wafer and the second wafer afterbonding the first wafer to the second wafer.